1. Field of the Invention
The invention relates to a process for preparing optionally substituted ODOP by reaction of optionally substituted o-phenylphenols with PCl.sub.3 in a first stage and hydrolysis of the product of the first stage in a second stage.
ODOPs are important additives for polymers for protection against oxidative degradation and for flame-proofing and further serve as starting materials for preparing further polymer additives.
2. Description of the Related Art
German Offenlegungsschrift 20 34 887, Example 5 and 6) discloses the preparation of 6-oxo-(6H)-dibenz-[c,e][1,2]-oxaphosphorins (ODOPs) by alkaline saponification of 6-chloro-(6H)-dibenz-[c,e][1,2]-oxaphosphorins with soda solution. In a cumbersome operation, this gives first the sodium salt of hydroxydiphenylphosphinic acid (III) in strongly contaminated aqueous-alkaline solution which has to be purified by means of activated carbon. ##STR1##
The sodium salt is convened by acidification into the free acid. This can, since it is sparingly soluble, be isolated from the aqueous salt solution. Subsequently, it has to be converted into the desired ODOP by healing to relatively high temperatures in vacuo. This is a very complicated and relatively environmentally unfriendly process, since it is associated with formation of salts and large amounts of wastewater.
Another reference (German Offenlegungsschrift 27 30 371, pp. 18 and 19) describes that 6-chloro-(6H)-dibenz-oxaphosphorin at 130.degree. C. is combined with a large amount of water, the hydrolysis is carded out, the water is distilled off under reduced pressure and the 6-oxo-dibenzoxaphosphorin is then obtained. The operation described is technically very problematical and can hardly be carried out on a large scale, since water vaporizes at 130.degree. C. and cools the reaction mixture. It is therefore necessary to work under increased pressure if the temperature of 130.degree. C. is to be maintained and thus uncontrolled crystallization is to be prevented, since the target compound melts significantly above the boiling point of water.
According to a further example (Example 3 in German Offenlegungsschrift 20 34 887), a crude product from the esterification of an o-phenylphenol with PCl.sub.3 is hydrolyzed by pouring onto ice. This gives the corresponding phosphinic acid which has to be separated from the water and, in a separate process step, be converted into the corresponding oxaphosphorin.
The processes mentioned give, as byproduct of the oxaphosphorin synthesis, dilute and contaminated hydrochloric acid as a byproduct which is difficult to use and to dispose of as well as salt solutions and wastewater.
None of the described processes for preparing ODOP makes any statements about the yield and the purity of the product. In each case it is only established that the desired product has been obtained.
Not once is it established whether the hydrolysis proceeds smoothly and uniformly or whether it is adversely affected by side-reactions or subsequent reactions. Comparison of the process proposals discussed brings those skilled in the art to the conclusion that obviously only the alkaline saponification leads to a pure material, since only here are a melting point and a matching phosphorus analysis presented. However, this conclusion is also based on the fact that the saponification proceeds via a plurality of process steps, namely the alkaline saponification, treatment with activated carbon, filtration, precipitation with hydrochloric acid, filtration with suction, washing with water, recrystallization of the isolated phosphinic acid from ethanol/water mixture and finally the ring closure to give the oxaphosphorin by dehydration at 150.degree. C./30 tort, associated with purification effects.
If the proposed processes are repeated, the yields and purities of ODOP found are not reproducible. In particular, the contamination of ODOP with o-phenylphenol fluctuates very widely and forces an additional purification of the ODOP obtained for it to be available in uniform quality. Our own studies indicated that the manner in which the o-phenylphenols are reacted with PCl.sub.3 has a great influence on the yield and purity of the ODOP.
For the reaction of o-phenylphenols with PCl.sub.3 too, there are various proposals.
According to German Offenlegungsschrift 20 34 887, PCl.sub.3 and o-phenylphenols are reacted with one another in stoichiometric amounts (p. 8, last section), i.e. a deficiency or excess should be avoided if possible, if only for different reasons.
However, in Example 1 of the abovementioned German Offenlegungsschrift, 1.2 mol of o-phenylphenol is reacted with an excess of PCl.sub.3, namely 1.5 mol. This is based on the escape of PCl.sub.3 together with the HCl eliminated. It is calculated that the specified excess of PCl.sub.3 is in actual fact carried out together with HCl during the reaction, i.e. a stoichiometric ratio of 1 mol of PCl.sub.3 to I mol of o-phenylphenol is finally present.
The yield of 6-chloro-(6H)-dibenz-[c,e][1,2]-oxaphosphorin (CDOP) formed here is not indicated. Repeating the reaction gives a yield of about 80% of the theoretical yield based on o-phenylphenol. This is confirmed by the studies in Phosphorus and Sulphur, Vol. 31, 71-76 (1987), particularly p. 74 (1). In that reference, a yield of 79% of the theoretical yield is given for the synthesis of ODOP according to German Offenlegungsschrift 20 34 887.
German Offenlegungsschrift 2 730 371 describes, in Example 1 (p. 18), a reaction of o-phenylphenol (40 mol) with phosphorus trichloride (47 mol) where, unlike Example 1 of German Offenlegungsschrift 20 34 887, the Lewis acid (here ZnCl.sub.2) is added from the beginning rather than only after the esterification of the o-phenylphenols with PCl.sub.3. Furthermore, PCl.sub.3 is added dropwise to the initially charged o-phenylphenol heated to 80.degree. and the mixture is then brought to 180.degree.. The further description of the experiment is imprecise and there is no indication of yield.
In EP 582 957, as in German Offenlegungsschrift 2 730 371, the catalyst is added to the reaction mixture right at the beginning and PCl.sub.3 is added dropwise, but the temperature is brought immediately to 180.degree. C. This gives a very good yield of CDOP.
In considering this prior art, those skilled in the art come to the conclusion that the combination of the proposals of German Offenlegungsschrift 2 730 371 and EP 582 957, namely the presence of the catalyst from the beginning of the reaction, the dropwise addition of PCl.sub.3 and the setting of a high temperature over the entire reaction time, effects the yield improvement found.
Experiments carried out observing these conditions give different yields depending on other important parameters, e.g. rate at which the PCl.sub.3 is added dropwise, rate at which the hydrogen chloride is given off and the temperature of the cooling medium in the reflux condenser. These parameters influence the amount of the PCl.sub.3 carried out with the hydrogen chloride.